The present invention relates to a hydraulic control system for an automatic transmission, and more particularly, to a hydraulic control system for an automatic transmission in which the supply of hydraulic pressure to two friction elements operating in different shift ranges is controlled by a single switch valve such that damage to the powertrain, which may result if two friction elements operate simultaneously, is prevented.
Conventional automatic transmissions used in vehicles typically include a torque converter, a powertrain realized through a multi-stage gearshift mechanism that is connected to the torque converter, and a hydraulic control system that selectively operates one of a plurality of operational elements of the powertrain according to a driving state of the vehicle.
In such an automatic transmission, although all the advantages of an automatic transmission over a manual transmission are provided (e.g., ease of driving), the generation of significant shift shock nevertheless remains a problem. To minimize shift shock, it is necessary to smoothly control clutches and brakes of the powertrain. In this regard, more effective than the most precise electronic control is the mounting of a one-way clutch. In the case where shifting is performed during an already ongoing shift process, good responsiveness can be expected with the use of a one-way clutch.
FIG. 4 shows a schematic view of a hydraulic control system for controlling a four-speed automatic transmission powertrain, which is capable of utilizing the advantages of one-way clutches during shifting between ranges 1 and 2, between ranges 3 and 4, and between range 4 and 2. With reference to the drawing, lines are formed to enable the supply of a D range pressure provided from a manual valve 200 to a first clutch C1 and to first, second, and third pressure control valves 202, 204, and 206; the supply of L range pressure provided from the manual valve 200 to the first pressure control valve 202; and the direct supply of R range pressure provided from the manual valve 200 to a third clutch C3 and to a first brake B1.
The D range pressure supplied to the first pressure control valve 202 is selectively supplied, according to control by a first solenoid valve 208, to an operational side of a second brake B2, and the L range pressure is supplied to the first brake B1 in a low L range. The first brake B1 is connected to the first pressure control valve 202 and an R range port of the manual valve 200 via a shuttle valve 210 such that hydraulic pressure is supplied to the first brake B1 no matter which direction hydraulic pressure is supplied from.
The D range pressure supplied to the second pressure control valve 204 is supplied to a second clutch C2 and the third pressure control valve 206 according to control by a second solenoid valve 212.
Also, the D range pressure supplied to the third pressure control valve 206 is selectively supplied to a fourth clutch C4 according to control by a third solenoid valve 214. In such an instance where the D range pressure is supplied to the fourth clutch C4, the third pressure control valve 206 supplies hydraulic pressure from the second pressure control valve 204 to a non-operational side of the second brake B2.
Hence, the first clutch C1 operates in first, second, and third speeds; the second clutch C2 operates in third and fourth speeds; the third clutch C3 operates in a reverse R range; the fourth clutch C4 operates in a park P range, the reverse R range, a neutral N range, and the low L range, and in the first, second, and third speeds according to driving conditions; the first brake B1 operates in the park P, reverse R, neutral N, and low L ranges; and the second brake B2 operates in the second and fourth speeds.
However, in the conventional hydraulic control system as described above, since the system simply acts to control line pressure and the solenoid valves merely operate as switch valves to control timing, precise shift control is not possible.
In particular, since control of non-operational sides of the second clutch C2 and the second brake B2 is linked, precise control is not possible shifting between ranges 2 and 3. Also, with the operation of the first brake B1 and the fourth clutch C4, which enable operation of the engine brake, since a method is used in which line pressure is directly supplied, significant shift shock may be generated.
Further, during manual shifting from the low 2 range to the low L range, the supply line pressure to the first brake B1 occurs simultaneously with the exhaust of operational-side pressure from the second brake B2, which also results in the generation of a shift shock. Also, manual shifting into the reverse R range from the drive D range when traveling at a high speed results in shifting being forcedly performed by line pressure, thereby causing shift shock as well as possible damage to friction material.
In addition, if manual control into the low L range is performed when driving in the third speed or higher, the second clutch C2 is disengaged such that engine fuel cut-off is performed at high speeds. As a result, an abrupt control into neutral occurs so that normal operation of the vehicle is not possible.
According to the present invention, there is provided a hydraulic control system for an automatic transmission, in which a powertrain is effectively and stably controlled by hydraulic pressure. The powertrain includes a first friction element, actuated to discontinue operation of a one-way clutch when an engine brake is needed, and a second friction element, which operates only when the first friction element is not engaged.
In a preferred embodiment of the present invention, the hydraulic control system comprises a manual valve and a switch valve. The manual valve receives hydraulic pressure from an oil pump and includes a forward range port for exhausting hydraulic pressure when driving in a forward range and an L range port for exhausting hydraulic pressure for low speed control. The switch valve is controlled by an engine brake signal pressure, solenoid pressure, and forward range pressure supplied from the forward range port. The switch valve selectively supplies control pressure to the first friction element and the second friction element.
According to a preferred embodiment of the present invention, the engine brake signal pressure is L range pressure supplied from the L range port of the manual valve. Preferably, in the selective supply of control pressure to the first and second friction elements by the switch valve, the control pressure is supplied to the second friction element only when either or both the engine brake signal pressure and the solenoid pressure is not being supplied. The switch valve includes a spool, and the forward range pressure acts on one side of the spool and the engine brake signal pressure and the solenoid pressure act on an opposite side of the spool. The spool generally includes a first land on which the forward range pressure acts, a second land on which the engine brake signal pressure acts, and a third land on which the solenoid pressure acts. Preferably one of either the second land or the third land has a surface area larger than a surface area of the first land and the other of either the second land or the third land has a surface area smaller than the surface area of the first land, and the difference between surface areas of the second land and the third land is smaller than the surface area of the first land.
According to a further embodiment of the present invention, the switch valve body includes an input port for receiving control pressure, a first supply port formed in a direction toward the first land from the input port and connected to the first friction element, and a second supply port formed in a direction toward the second land from the input port and connected to the second friction element. Further, the fourth and fifth lands, which provide selective communication between the input port and the first supply port or the second supply port, are formed on either side of the input port. Preferably, the valve body further includes a first exhaust port for exhausting hydraulic pressure from the first supply port, and a second exhaust port for exhausting pressure from the second supply port.
In an alternative embodiment of the invention, the hydraulic control system includes at least two switch valves for facilitating control of plural friction elements of the transmission. A first switch valve communicates with hydraulic lines of the system to provide selective supply of hydraulic fluid pressure to a first and a fourth clutch from the drive range line pressure. A second switch valve communicates with hydraulic lines of the system to provide selective supply of hydraulic fluid pressure to a first brake and second clutch from the neutral range line pressure. The control system also preferably includes a manual valve responsive to a user manipulated shift lever for selecting between different available shift ranges of the transmission, such as the drive, neutral, reverse and park ranges. Also in a preferred embodiment one solenoid valve may provide one source of control pressure to both the first and second switch valves.